Image intensifiers are well known for their ability to enhance night-time vision. An image intensifier amplifies the incident light received by it to produce a signal that is bright enough for detection by the eyes of a viewer. These devices, which are particularly useful for providing images from dark regions, have both industrial and military application. The U.S. military uses image intensifiers during night-time operations for viewing and aiming at targets that otherwise would not be visible. Low intensity visible spectrum radiation and near-infrared radiation is reflected from a target, and the reflected energy is amplified by the image intensifier. As a result, the target is made visible without the use of additional light. Other examples include using image intensifiers for enhancing the night vision of aviators, for providing night vision to sufferers of retinitis pigmentosa (night blindness), and for photographing astronomical bodies.
FIG. 1 depicts an exemplary image intensifier 10. The image intensifier 10 includes an objective lens 12 that focuses visible and infrared radiation (collectively referred to herein as light) from a distant object onto a photocathode 14. The photocathode 14, e.g., a photoemissive semiconductor heterostructure that is extremely sensitive to low-radiation levels of light in the 580-900 nm spectral range, provides a spatially coherent emission of electrons in response to the electromagnetic radiation. Electrons emitted from the photocathode 14 are accelerated towards an input plane of a micro-channel plate (MCP) 20. The MCP 20 amplifies the incident electrons in a spatially coherent manner. Electrons emerging from an output plane of the MCP 20 are accelerated toward a phosphor screen 16 (anode), which is maintained at a higher positive potential than the output of the MCP 20. The phosphor screen 16 converts the emitted electrons into visible light. An operator may view the visible light image provided by the phosphor screen through an eyepiece 18.
Conventional MCPs 20 include a thin glass plate having an array of microscopic holes through it used to increase the density of the electron emission from the photocathode 14. Electrons impinging on interior sides of the holes through the MCP 20 result in the emission of a number of secondary electrons each of which, in turn, causes the emission of more secondary electrons. Thus, each microscopic hole acts as a channel-type secondary emission electron multiplier having a gain of up to, for example, ten thousand. The electron gain of the MCP 20 is controlled primarily by the potential difference between its input and output planes. A power source 22 applies power to the photocathode 14, the MCP 20, and the phosphor screen 16.
Image intensifiers for use in night vision systems commonly use a measurement called Figure of Merit (FOM) for image quality. FOM is the arithmetic product of the resolution, measured in line pairs per millimeter (lp/mm) and signal-to-noise ratio (SNR), which is unitless. Resolution typically varies in the range of 50 to 72 lp/mm. SNR typically varies in the range of 20 to 25. So FOM typically varies in the range of 1,000 to 1,800, with a higher FOM generally representing a superior overall image quality. FOM may be important in some contexts because the United States government regulates the export of night vision systems by requiring that exported items have a FOM below a specified threshold. Accordingly, methods and apparatus of regulating FOM of an image intensifier are useful.